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#include <condition_variable>
#include <iostream>
#include <mutex>
#include <thread>

constexpr size_t numThreads = 5;

std::mutex mutex;
std::condition_variable cond[numThreads];

void run_step(size_t myStep) {
  static size_t currentStep = 0;
  std::unique_lock<std::mutex> lk(mutex);

  std::cout << "Thread " << myStep << " has the lock" << std::endl;

  while (currentStep != myStep) {
    std::cout << "Thread " << myStep << " is sleeping..." << std::endl;
    cond[myStep].wait(lk);
    std::cout << "Thread " << myStep << " woke up" << std::endl;
  }

  // Do processing ...
  std::cout << "Thread " << myStep << " is processing..." << std::endl;
  currentStep++;

  // Signal next step thread.
  if ((myStep + 1) < numThreads) {
    cond[myStep + 1].notify_one();
  }

  std::cout << "Thread " << myStep << " is exiting..." << std::endl;
}

int main() {
  std::thread threads[numThreads];

  // Create threads...
  for (size_t i = 0; i < numThreads; ++i) {
    threads[i] = std::thread(run_step, i);
  }

  // Wait for all threads to complete.
  for (size_t i = numThreads; i != 0; --i) {
    threads[i - 1].join();
  }

  return 0;
}main() unchanged ...